DC power supply utilizing real time estimation of dynamic impedance

ABSTRACT

There is provided by this invention an apparatus and method for controlling a dc magnetron plasma processing system that automatically adjusts the control signal to the power supply based upon the dynamic impedance of the load to control the output power to the plasma. The output voltage and the output current of the power supply that supplies power to the plasma is sampled over at a sampling frequency at least four to five times higher than the switching frequency and the dynamic impedance of the plasma is calculated based upon the sampled voltage and current from the algorithm 
               R   plasma     =       Δ   ⁢           ⁢     V   n         Δ   ⁢           ⁢     I   n               
wherein ΔV n  and ΔI n  is the maximum difference among samples on one switching cycle. If the dynamic impedance seen is negative in nature then the control signal is compensated accordingly.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to any controller for a power supplythat utilizes a compensator to shape the response of the control loop toexternal disturbances, and more particularly to a control means forcalculating the dynamic impedance of a DC magnetron based process.

2. Brief Description of the Prior Art

In plasma processing for the manufacture of thin films for integratedcircuits, flat panel displays, glass coatings, etc a fast controller isrequired to effectively control the power delivered to a wide range ofplasma processes. Controllers are designed accounting for the control tooutput transfer function of the power supply. The transfer function ofthe power supply depends on the dynamic impedance of the load. Theprimary function of a controller for a power supply is to achieve andmaintain any commanded control signal. The controller is designedaccounting for the control to output, and line to output transferfunctions of the power supply. The dependence of the controller on loadimpedance may be in the form of DC gain, or the location of a pole or azero in the transfer function of the power supply. Any change in theoutput impedance can significantly influence the performance of thecontrol loop and sometimes even catastrophically by making a previouslystable system unstable. As shown in FIG. 1 the dynamic impedance ispositive for curves A and B and negative for curve C. The voltage andresistance are a function of the operating point and can change withtime. FIG. 2 illustrates how depending on the process and plasmacharacteristics the plasma load can be modeled as a voltage source inseries with a resistance. In the cases where variations in the transferfunction due to a change in the dynamic impedance may be limited to theDC gain and could be easily compensated with analog circuitry or digitalgain blocks, it is performed by a priori analysis of the transferfunctions and implementing lookup tables for different load conditions.

Some controllers utilize DC current and the DC voltage to calculate theDC impedance of the plasma. However, this method has a disadvantage,since it assumes that the plasma represents a load that is only animpedance in nature. In the case of model plasma as shown in FIG. 2 thenan approach is to use an empirical value of the plasma voltage andsubtract this from the DC voltage and then use this to calculate theactual dynamic impedance of the plasma. This approach has a disadvantagebecause there are wide ranges of internal voltages that even the sameplasma can exhibit.

For instance, U.S. Pat. No. 5,543,689 issued to Ryusuke Ohta et aldiscloses a high frequency power source wherein the controller has amemory for storing initial plasma characteristic data and plasma gain, acomparable operation section for calculating control target data frominitial plasma characteristic data and detected power data and computingthe control gain data from the difference between the control targetdata and the power control signal data. The control target data isderived by subtracting the initial plasma data from the detected plasmadata. However, the process described relies upon an alarm for manualadjustment of the control signal. Such a system has a slow response timeand may cause the process to shut down before correction can be made.

These types of control schemes are undesirable because 1) estimation ofthe plasma resistance is extremely difficult; 2) implementation of thecompensation is limited to the DC gain; 3) they are not continuous anddependence may not be monotonic with respect to the load impedance; and4) the variation in the poles and zeros of the transfer function withthe load impedance may still degrade the performance of the system andin some instances may also cause the system to go unstable. Also, fasterloop speeds require ability to compensate for load and create transferfunctions to create a fairly wide-band system.

It would be desirable if there were provided a controller for a DC powersupply that utilized a fast control loop which works for a wide range ofplasma processes. It would also be desirable if there were providedmethod of control which enables adaptive and non-linear control byestimating dynamic impedance of the load.

SUMMARY OF THE INVENTION

There is provided by this invention a controller for a power supply suchas a power supply for a DC magnetron process system that utilizes ananalog to digital converter (ADC) in combination with a digitalmicroprocessor to estimate the dynamic impedance of the plasma. Anautomatic control loop maintains the control signal for the power supplyby sampling the output voltage and current of the power supply tocorrect for differences between the output and a predetermined controlsignal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the V-I curves for different plasma loads;

FIG. 2 illustrates schematically a model of a plasma load;

FIG. 3 illustrates a block diagram of the controller and power supplyincorporating the principles of this invention;

FIG. 4 illustrates waveforms for real time estimation of the dynamicimpedance from transient voltage and current waveforms;

FIG. 5 illustrates a flow chart for the algorithm used for estimation ofdynamic impedance;

FIG. 6 illustrates waveforms for estimation of the dynamic impedancefrom the switching ripple; and

FIG. 7 illustrates a flow chart for the algorithm used for estimation ofthe dynamic impedance from the switching ripple.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 3 there is shown a plasma processing system 10 such asa DC magnetron processing system that incorporates the principles ofthis invention. A plasma chamber 12 has contained therein electrodes 14and 16. A power supply 18 supplies a voltage to the electrodes 14 and 16in order to ignite plasma 20 in a reactive gas (not shown). Particlesfrom the plasma are disposed to deposit a thin film on a substrate (notshown) in the chamber. To compensate for changes in the output of thepower supply 18 a controller 22 automatically adjusts the control signal24 to the power supply based upon the change of the dynamic impedance ofthe plasma 20. The controller is generally comprised of an analog todigital converter 26 and microprocessor 28. The A/D converter samplesthe output voltage and current of the power supply and the values areinputted to the microprocessor which makes a real time estimation of thedynamic impedance of the plasma and sends a control signal 24 to thepower supply to make output adjustments as necessary.

FIG. 4 illustrates the typical transient to control signalpost-plasma-ignition for the following algorithm:

$R_{plasma} = \frac{\Delta\; V_{n - 1}}{\Delta\; I_{n - 1}}$

Where ΔV_(n-1) represents the moving increment in plasma voltage over afixed number of samples based upon the sampling frequency. ΔI_(n-1)represents the same value for the plasma current. This occurs when theabsolute value of the increments in voltage and current are lower thanthe threshold. A predetermined threshold is chosen to estimate when theplasma reaches steady state. This threshold is chosen considering thesampling frequency and the power system dynamics. This enables thecalculation of the dynamic impedance when the plasma is just about toreach the set point. It is to be noted that this algorithm enables thecontroller to estimate if the dynamic impedance being seen by the powersupply is negative in nature, and compensate appropriately for negativeimpedance.

FIG. 5 illustrates the flow chart used for estimating the dynamicimpedance of the plasma. In the first step, depending on the samplingfrequency, the A/D converter samples both the output voltage and currentof the power supply. This method calculates the dynamic impedance priorto the plasma reaching the set point and not while the plasma is at setpoint. In order to measure the impedance at set point another digitalmeasurement system utilizes the A/D converter sampling at a frequencyhigher than the power supply switching frequency ripple seen on theplasma current and voltage, which is usually the switching frequency ora multiple of it. This data can be used to estimate the dynamicimpedance of the plasma. It is specifically possible to determinewhether the dynamic impedance is positive or negative with thisalgorithm.

FIG. 6 illustrates the estimation of the dynamic impedance from theswitching ripple. In order to accomplish this task the samplingfrequency is usually on the order of four or five times the ripple onthe voltage and current. Then the maximum and minimum values of thevoltage/current within an integral number of switching periods areestimated and the difference between them gives ΔV_(n) and ΔI_(n).

It is then possible to look for the maximum change in either the voltageor current and then use the change in the other parameter (current orvoltage respectively) during the same time interval to calculate thedynamic impedance of the plasma.

$R_{plasma} = \frac{\Delta\; V_{n}}{\Delta\; I_{n}}$

FIG. 7 illustrates the flow chart for calculating the dynamic impedancefrom the switching ripple. The A/D converter acquires a new set of nsamples for voltage and current. The microprocessor calculates themaximum and minimum of the given samples and the difference between themand assigns to delta V and delta I. The dynamic impedance is thencalculated from these values. This algorithm specifically determineswhether the dynamic impedance is negative or positive.

It must be noted here that the impedances calculated with the twomethods are at two different frequencies. The one measured with thetransient set up is at a frequency much closer to the control loopbandwidth. The one calculated using the switching frequency ripple is ata much higher frequency than the control loop bandwidth. However, theimpedance estimated using the switching ripple gives insight into thehigh-frequency performance of the plasma, and is useful for determiningthe fast dynamics of the plasma.

Although there is illustrated and described specific structure anddetails of operation, it is clearly understood that the same were merelyfor purposes of illustration and that changes and modifications may bereadily made therein by those skilled in the art without departing fromthe spirit and the scope of this invention.

1. A method of controlling a dc magnetron plasma processing systemconsisting of the steps of: sampling the output voltage and the outputcurrent of the power supply that supplies power to the plasma over anumber of samples based upon a sampling frequency; calculating thedynamic impedance of the plasma based upon the sampled voltage andcurrent from the algorithm$R_{plasma} = \frac{\Delta\; V_{n - 1}}{\Delta\; I_{n - 1}}$ wherebyΔV_(n-1) and ΔI_(n-1) are calculated from samples just before plasmagets to the steady-state condition; estimate if the dynamic impedanceseen is negative in nature and compensate accordingly and sending acontrol signal to adjust the control signal of the power supply basedupon the dynamic impedance of the plasma.
 2. A method of controlling adc magnetron plasma processing system consisting of the steps of:sampling the output voltage and the output current of the power supplythat supplies power to the plasma at a sampling frequency at least fourto five times higher than the switching frequency; calculating thedynamic impedance of the plasma based upon the sampled voltage andcurrent from the algorithm$R_{plasma} = \frac{\Delta\; V_{n}}{\Delta\; I_{n}}$ wherein ΔV_(n) andΔI_(n) are the maximum differences among samples on one switching cycle;estimating from this method if the dynamic impedance is negative andthen compensating accordingly and sending a control signal to adjust thecontrol signal of the power supply based upon the dynamic impedance ofthe plasma.